The invention relates to a method for monitoring a system comprising a number of reading devices and a plurality of portable communication units.
From the prior art there are known systems with which utilized services are captured via a data exchange between a reading device and a portable communication unit carried by the user, so that these services can be billed to the user at a later time. Systems for capturing services are known in particular from the field of public transport. Via corresponding reading devices in means of transport, which communicate with the communication units of users, the travelled paths are captured here.
In the just described systems there are often employed RFID tags as portable communication units, which contactlessly communicate with a corresponding reading device. In many applications there are used UHF-RFID tags which can be queried by a corresponding RFID reading device and send back their identity by modulation of the high-frequency energy scattered back at the antenna of the tag (backscatter). As UHF tags have no power supply of their own, they are normally of a simple construction and only insufficiently protected against forgery.
In the print A. Arbit, J. Oron, A. Wool, Toward Practical Public Key Anti-Counterfeiting for Low-Cost EPC Tags, 2011, IEEE International Conference on RFID, there is described a protocol based on a public key cryptography for the secured identification of RFID tags. This protocol prevents the creation of new tags, but does not protect against the manufacturing of duplicates of already existing tags.
In the print U.S. Pat. No. 6,766,161 B2 there is described a method for recognizing duplicated communication devices based on transaction codes. Here, in a host and a communication device there is held a sequence of consecutive transaction codes, upon each communication between host and communication device there being transmitted a new transaction code of the sequence to the host. The host then checks whether the transmitted transaction code matches the transaction code which is to be expected according to the sequence deposited at the host.